The Associated Press called the race for Democratic challenger Harley Rouda on Saturday night. With 100% of precincts reporting, Mr. Rouda had 52% of the vote to Mr. Rohrabacher's 48%.

First elected to Congress in 1988, Mr. Rohrabacher represents a district in Orange County, Calif., traditionally a bastion of conservatism in a state that has otherwise trended Democrat. The win by Mr. Rouda, a real-estate developer, solidifies Democrats’ statewide gains in a longtime Republican stronghold, and comes as the party regained control of the House in the midterms.

Mr. Rouda argued that Mr. Rohrabacher was out of step with the district.

Mr. Rohrabacher is one of the most colorful and iconoclastic members of Congress. Known in the 1980s for activism on behalf of Afghan rebels fighting the Soviets, he has become in recent years an advocate for closer ties between the U.S. and Russia. He is one of the few Republicans to back legal marijuana.

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AP RACE PROJECTION

Katie Hill (D) Wins California House District 25

Ahead of Election Day, we selected counties across the U.S. that typically reflect how their state votes in Senate races. With most of those races now decided, here’s a look at how those counties fared relative to the overall vote.

FLORIDA: Nelson vs. Scott

In Monroe County, Fla., we were tracking the senate race between Democratic incumbent Sen. Bill Nelson and Republican challenger Rick Scott, currently the state’s governor. Both the county (dotted line, with final result) and the overall vote (solid line, plotted through the night), remained very much in sync -- and both leaned slightly red. That reflected the very tight nature of the race, which remained too close to call as of Thursday, with Mr. Scott leading narrowly by 50.1% to 49.9%.

MISSOURI: McCaskill vs. Hawley

In Platte County, Mo., we were tracking the race between incumbent Democratic Sen. Claire McCaskill and Republican challenger Josh Hawley. Mr. Hawley won the county by less than 200 votes , or about 0.3%, which is why the dotted line is only slightly into red territory. But he won the state more comfortably, by six percentage points, and is headed to Washington next year.

ARIZONA: Sinema vs. McSally

Arizona is one of the two remaining uncalled Senate races, with Democrat Kyrsten Sinema neck and neck with Republican Martha McSally with several hundred thousand votes yet to be counted. Our bellwether, Yuma County, leaned solidly Republican.

Montana: Tester vs. Rosendale

In Montana, Republican challenger Matt Rosendale was hoping to unseat Democratic Sen. Jon Tester in this solidly red state. Mr. Tester ran strongly in our bellwether county of Park, Mont., and while the statewide total briefly dipped into the red, it finished in the blue with room to spare, 50.1% to 47%.

TEXAS: Cruz vs. O'Rourke

Republican Sen. Ted Cruz faced Democratic challenger Beto O’Rourke in the most closely watched race of the night. Texas is a deep red state, and no Democrat has held a Senate seat there since 1993, but Mr. O’Rourke’s run had generated national interest as polls showed him within striking distance. While our bellwether county of Tarrant went narrowly for Mr. O’Rourke, the statewide total went for Mr. Cruz, 50.9% to 48.3%, and he retained his seat.

NEVADA: Heller vs. Rosen

In Nevada, Democrat Jacky Rosen was looking to oust Republican Sen. Dean Heller, seen as one of the most vulnerable GOP incumbents in 2018. Our bellwether, Washoe County, ended up in line with the statewide total -- a win for Ms. Rosen of about five points.

NORTH DAKOTA: Heitkamp vs. Cramer

Democratic Sen. Heidi Heitkamp faced an uphill battle in holding onto her seat in a state President Trump won by more than 30 points. While she fared better than Hillary Clinton did in 2016, she still lost to Republican challenger Kevin Cramer by 10 points statewide, 55.4% to 44.6%, and by slightly more in our bellwether Griggs County.

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Tensions Rise and Accusations Fly as Florida Races Hang in the Balance

Senate, House Seats Remain Unclaimed as Vote Counting Continues

When Democrat Lucy McBath unseated Republican incumbent Karen Handel in Georgia’s sixth congressional district Thursday, she added to the Democrats’ new majority in the House with more than a dozen contests still undecided.

In the Senate, the situation is reversed, with Republicans clinching control on Tuesday night and looking to three outstanding races to pad their margin.

We may be waiting a while for the results: For races in California, Arizona, Georgia, and elsewhere, it may be days or even weeks before election officials provide a final tally.